Data from: Nitrogen addition and warming modulate the pathogen impact on plant biomass by shifting intraspecific functional traits and reducing species richness

1. Foliar fungal pathogens can substantially reduce plant biomass. This
effect can be modulated by environment conditions, such as soil nitrogen
availability and air temperature. The ongoing global changes are altering
these variables and thus interact with pathogens to influence plant
biomass, but experimental test of their interactions is scarce. 2. We
conducted a 4-year field experiment in a Tibetan alpine meadow to examine
the interactive effects of nitrogen addition, warming and foliar pathogens
(via fungicide application) on plant biomass. We also measured plant
functional traits, species richness and abundance to test the possible
mechanisms underlying these interactions. 3. Our results showed that
foliar fungal pathogens reduced plant community biomass under nitrogen
addition, which in turn weakened the positive nitrogen effect on community
biomass. Mechanistically, nitrogen addition shifted the plant communities
towards fast-growing traits; this happened predominantly because of
changes in within-species trait values, including an increase in specific
leaf area and height. These trait changes resulted in greater suppression
of plant biomass by pathogens, likely because of the trade-offs associated
with the allocation of resources to plant growth and defense. Moreover,
the reduction in species richness amplified the pathogen effect under
nitrogen addition due to the increased density and susceptibility of the
most dominant species (i.e. Kobresia capillifolia). Furthermore, warming
did not interact with pathogens and nitrogen addition to influence plant
community biomass, but their three-way interaction modified the biomass of
K. capillifolia. Specifically, warming enhanced the positive effect of
nitrogen addition on the biomass of K. capillifolia in the fungicide, low
infection plots, while it weakened the nitrogen effect in the no
fungicide, high infection plots. 4. Synthesis: Our results demonstrate how
pathogens interact with nitrogen addition and warming to influence the
biomass of dominant species and the whole plant community. Our study
highlights the importance of considering foliar fungal pathogens when
assessing ecosystem responses to multiple global change factors.